Differential parasitism by Naobranchia occidentalis and Nectobrachia indivisa on northern rock sole and southern rock sole in Alaskan waters - Copepoda: Naobranchiidae - Copepoda: Lernaeopodidae - Lepidopsetta polyxystra Orr and Matarese, 2000 - L. bilineata Ayres, 1855 - Statistical Data Included

Fishery Bulletin, April, 2001 by Mark Zimmermann, Robin C. Harrison, Anthony F. Jones

Nectobrachia indivisa also selectively infested larger northern rock sole but rarely infested southern rock sole, suggesting an apparent species preference. Because N. indivisa attaches by means of a bulla permanently embedded near the end of a gill filament, it seems less likely that this parasite would be limited by mechanical restrictions such as the diameter of the gill filament. The parasite Salmincola californiensis, which also attaches to its host by means of a bulla (Kabata and Cousens, 1972), seemed to be dependent on its sockeye salmon (Oncorhynchus nerka) host reaching a large size before the gill filaments became the preferred site of attachment (Kabata and Cousens, 1977). Perhaps N. indivisa is able to infest northern rock sole successfully only after the fish reach a certain size.

We did not perform histological analysis of gill filaments to determine differential damage caused by infestation of Naobranchia occidentalis and Nectobrachia indivisa. Roubal (1999) determined that damage caused by Naobranchia variabilis was restricted to the infested gill filament and was minor. Kabata (1984) noted that naobranchids only partially compress the gill filament, do not completely restrict blood flow, and do not cause whitening of gill filaments. Roubal (1999) also determined that N. variabilis live by feeding on the blood supply and not by grazing the tissue, as do lernaeopodids. Kabata and Cousens (1977) reported macroscopic observations of significant atrophy and tissue reaction to as much as one-third of the gill filament surface area of juvenile sockeye salmon due to the presence of S. californiensis. This type of damage is consistent with our macroscopic observations of the white gills of northern rock soles seen early in the Gulf of Alaska survey. Kabata and Cousens (1977) also reported on microscopic observations of gill filament damage, sometimes including proliferation of the gill epithelium, hypertrophy of epithelial cells, fusion of adjacent filaments, thickening of filament walls, blood blisters, and erosion of epithelial cells as the oral apparatus of the S. californiensis scrapes them for ingestion.

Margolis and Arthur (1979), Kabata and Whitaker (1984), and Kabata (1988) all reported that both Naobranchia occidentalis and Nectobrachia indivisa occurred on rock soles in Pacific Canadian waters, prior to the determination that there are two species of rock sole in this area (Orr and Matarese, 2000). Neither Moles (1982) nor Love and Moser (1983) reported these parasites from rock soles from Alaskan waters and U.S. west coast waters, respectively. Our study reported new host records for Acanthochondria vancouverensis and Haemobaphes diceraus on northern rock sole.

The apparent preference of both parasites for the northern rock soles is supported by the conclusion of Orr and Matarese (2000) that northern and southern rock soles are two distinct species. Possible differences in the ecology of these newly described rock sole species, such as food habits, growth rates, habitats, spawning seasons and locations, nursery grounds, and seasonal and ontogenetic migrations might also account for the differential parasitism that we observed. It is not known if parasite prevalence and intensity is related to differences in the ecology and behavior of these closely related species, or related to differences in their anatomy or physiology. As more research is done on these species, based on the work of Orr and Matarese (2000), more potential differences will be determined.